The injection molded Pet Preform mold consists of a dynamic mold and a static mold. The movable mold is fixed on the movable plate of the injection molding machine, and the fixed mold is fixed on the fixing plate of the injection molding machine. During injection molding, the moving and stationary molds close and form a feed system and cavity. When the mold is opened, the movable mold and the static mold are separated for taking out the plastic product.
The mold structure varies depending on the type and performance of the plastic, the shape and structure of the plastic product, and the type of injection molding machine, but the basic structure is the same. The mold is mainly composed of a feeding system, a temperature regulating system, a molded part and a structural part. Feed systems and molded parts are parts that come into direct contact with plastic and vary with plastics and products. On plastic molds, they are the most complex and varied parts with the highest demands on processing gloss and precision.
The feed system is the portion of the flow path through which the plastic enters the cavity from the nozzle. It includes main runners, cold section plugs, sub runners and sprues. The molded part is an element that forms a shape of the product, such as a movable mold, a fixed mold, a cavity, a core, a molded rod, and a vent.
1. Feeding system
The feed system is also known as the runner system. This is a set of feed channels that direct the plastic melt from the injection molding machine to the mold cavity. It is mainly composed of mainstream roads, auxiliary runners, gates and cold section plugs. It is directly related to the molding quality and production efficiency of plastic products.
Main runner
This is a passage that connects the nozzle of the injection molding machine to the sub-flow path or cavity. The top of the main wheel is concave for easy connection to the nozzle. The inlet diameter of the main flow channel should be slightly larger than the nozzle diameter (0.8 mm) to avoid spillage and to prevent blockage due to inaccurate connections. The diameter of the inlet depends on the size of the product. Usually 4 – 8 mm. The main diameter should be extended inward by a 3 to 5 degree angle to help the sprue margin to be demolded.
B cold section
This is a hole at the end of the main runner that collects the cold material left between the injection molding ends at the end of the nozzle to prevent blockage of the sub-gate or gate. If cold waste is mixed into the cavity, it can cause internal stress in the product. The cold section plug has a diameter of about 8 – 10 mm and a depth of 6 mm. In order to facilitate demolding, the bottom is usually supported by a knockout pin. The top of the knockout pin is preferably designed as a meandering hook or a recess is provided thereon. Then the main runner allowance will be smoothly pulled out when demoulding.
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